The invention relates to agents that inhibit hepatitis C virus (HCV) RNA-dependent RNA polymerase (RdRp). The invention also relates to the use of such compounds in pharmaceutical compositions and therapeutic treatments useful for inhibition of HCV replication.
HCV is an enveloped RNA virus containing a single-stranded positive-sense RNA genome approximately 9.5 kb in length (Choo, et al., Science 244:359-362 (1989)). The RNA genome contains a 5′-nontranslated region (5′ NTR) of 341 nucleotides (Brown, et al., Nucl. Acids Res. 20:5041-5045 (1992); Bukh, et al., Proc. Natl. Acad. Sci. USA 89:4942-4946 (1992)), a large open reading frame (ORF) encoding a single polypeptide of 3,010 to 3,040 amino acids (Choo, et al. (1989), supra;), and a 3′-nontranslated region (3′-NTR) of variable length of about 230 nucleotides (Kolykhalov, et al., J. Virol. 70:3363-3371 (1996); Tanaka, et al., J. Virol. 70:3307-3312 (1996)).
The 5′ NTR is one of the most conserved regions of the viral genome and plays a pivotal role in the initiation of translation of the viral polyprotein. A single ORF encodes a polyprotein that is co- or post-translationally processed into structural (core, E1, and E2) and nonstructural (NS2, NS3, NS4A, NS4B, NS5A, and NS5B) viral proteins by either cellular or viral proteinases (Bartenschlager (1997), supra). The 3′ NTR consists of three distinct regions: a variable region of about 38 nucleotides following the stop codon of the polyprotein, a polyuridine tract of variable length with interspersed substitutions of cystines, and 98 nucleotides (nt) at the very 3′ end which are highly conserved among various HCV isolates. The order of the genes within the genome is: NH2—C-E1-E2-p7-NS2-NS3-NS4A-NS4B-NS5A-NS5B—COOH (Grakoui, et al., J. Virol. 67:1385-1395 (1993)).
Hepatitis C virus (HCV) is a member of the hepacivirus genus in the family Flaviviridae. It is the major causative agent of non-A, non-B viral hepatitis and is the major cause of transfusion-associated hepatitis and accounts for a significant proportion of hepatitis cases worldwide. Although acute HCV infection is often asymptomatic, nearly 80% of cases resolve to chronic hepatitis. The persistent property of the HCV infection has been explained by its ability to escape from the host immune surveillance through hypermutability of the exposed regions in the envelope protein E2 (Weiner, et al., Virology 180:842-848 (1991); Weiner, et al. Proc. Natl. Acad. Sci. USA 89:3468-3472 (1992).
Processing of the structural proteins core (C), envelope protein 1 and (E1, E2), and the p7 region is mediated by host signal peptidases. In contrast, maturation of the nonstructural (NS) region is accomplished by two viral enzymes. The HCV polyprotein is first cleaved by a host signal peptidase generating the structural proteins C/E1, E1/E2, E2/p7, and p7/NS2 (Hijikata, et al., Proc. Natl. Acad. Sci. USA 88:5547-5551 (1991); Lin, et al., J. Virol. 68:5063-5073 (1994)). The NS2-3 proteinase, which is a metalloprotease, then cleaves at the NS2/NS3 junction. The NS3/4A proteinase complex (NS3 serine protease/NS4A cofactor), then at all the remaining cleavage sites (Bartenschlager, et al., J. Virol. 67:3835-3844 (1993); Bartenschlager, (1997), supra). RNA helicase and NTPase activities have also been identified in the NS3 protein. The N-terminal one-third of the NS3 protein functions as a protease, and the remaining two-thirds of the molecule acts as a helicase/ATPase, which is thought to be involved in HCV replication (Bartenschlager, (1997), supra). NS5A may be phosphorylated and act as a putative cofactor of NS5B. The fourth viral enzyme, NS5B, is an RNA-dependent RNA polymerase (RdRp) and a key component responsible for replication of the viral RNA genome (Lohmann, et al., J. Virol. 71:8416-8428 (1997)).
Replication of HCV is thought to occur in membrane-associated replication complexes. Within these, the genomic plus-strand RNA is transcribed into minus-strand RNA, which in turn can be used as a template for synthesis of progeny genomic plus strands. Two viral proteins appear to be involved in this reaction: the NS3 protein, which carries in the carboxy terminal two-thirds a nucleoside triphosphatase/RNA helicase, and the NS5B protein, which is a membrane-associated phosphoprotein with an RNA-dependent RNA polymerase activity (RdRp) (Hwang, et al., J. Virol. 227:439-446 (1997)). While the role of NS3 in RNA replication is less clear, NS5B apparently is the key enzyme responsible for synthesis of progeny RNA strands. Using recombinant baculoviruses to express NS5B in insect cells and a synthetic nonviral RNA as a substrate, two enzymatic activities have been identified as being associated with NS5B. The two activities include a primer-dependent RdRp and a terminal transferase (TNTase) activity. NS5B's activity was confirmed and further characterized through the use of the HCV RNA genome as a substrate (Lohmann, et al., Virology 249:108-118 (1998)). Recent studies have shown that NS5B with a C-terminal 21 amino-acid truncation expressed in Escherichia coli is also active for in vitro RNA synthesis (Ferrari, et al., J. Virol. 73:1649-1654 (1999); Yamashita, et al., J. Biol. Chem. 273:15479-15486 (1998)).
Since persistent infection of HCV is related to chronic hepatitis and eventually to hepatocarcinogenesis, HCV replication is one of the targets to eliminate HCV reproduction and to prevent hepatocellular carcinoma. Unfortunately, present treatment approaches for HCV infection are characterized by relatively poor efficacy and an unfavorable side-effect profile. Therefore, intensive effort is directed at the discovery of molecules to treat this disease, including the discovery of drugs designed to inhibit HC replication, as there is a persistent need for non-peptide, small-molecule compounds that are HCV RdRp inhibitors having desirable or improved physical and chemical properties appropriate for pharmaceutical applications.